A copper sphere is suspended in a evacuated chamber maintained at 300 K. The sphere is maintained at constant temperature of 900 K by heating electrically. A total of 300 W electric power is needed to do this. When half of the surface of the copper sphere is completely blackened, 600 W is needed to maintain the same temperature of sphere. The emissivity of copper is

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Detailed Solution

$300 = e σ A (900^{4} - 300^{4})$ … (i)
$600 = \frac{σ A}{2} (900^{4} - 300^{4}) + \frac{e σ A}{2} (900^{4} - 300^{4})$ … (ii)
$e = \frac{1}{3}$

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